What is the Half Power Beamwidth (3dB Angle) of an Antenna?

Half power beamwidth of an antenna is the angular separation, in which the magnitude of the radiation pattern decreases by 50% (or -3dB) from the peak of the main beam. It is the area where most of the power is radiated.

This calculator calculates the HPBW angle when the spot size in meter and distance from antenna and the equipment under test is given to the input.

A half-power beamwidth angle can be determined by the angle between the two lines drawn from the origin of the main lobe and intersecting the half power points of the main lobe. 

The unit of the HPBW is radians or degrees.